In order to achieve higher density semiconductor circuits, it is desired that higher optical-resolution lithographic light sources be developed. Since resolution scales linearly with wavelength, many in the semiconductor industry view extreme ultraviolet lithography (EUVL) technology as a promising technology that in coming years will be used to produce smaller and faster microchips with feature sizes of 32 nm or less.
Several issues remain to be addressed before EUVL can be successfully applied in high volume semiconductor production. One is the need to develop a high-power, long-lifetime EUVL light source. Extreme ultraviolet light (EUV) is essentially “soft X-ray” emission, and light sources involving the generation of laser-produced plasmas (LPPs) have been one of the most promising candidates for providing such emissions. Indeed, recent international efforts have resulted in great progress in enhancing the conversion efficiency achieved in such light sources.
EUVL light sources can employ a high repetition rate laser (10-100 kHz) with 100-1000 mJ pulse energy, and operate by irradiating a metal target with the high-power laser radiation to cause the target material to be vaporized into a plasma with excited metal atoms and ions. The excited metal atoms and ions in turn emit the desired soft X-rays, which are then collected and transported onto a photoresist coated wafer. Further detailed information regarding the design of such light sources can be obtained in “Extreme ultraviolet light sources for use in semiconductor lithography—state of the art and future development” by Uwe Stamm (J. Phys. D: Appl. Phys. 37 (2004) 3244-3253), which is hereby incorporated by reference herein.
Notwithstanding the promise of such light sources, a remaining significant problem in implementing EUVL light sources is the generation of energetic debris from the plasmas, which can damage the optics in a EUVL light source. For example, while solid density tin targets offer the highest in-band conversion efficiency and the simplest target supply for high repetition rate operation, such targets result in high kinetic energy debris and subsequent optic damage that limits the source lifetime.
Various attempts have been made to solve the problem of fast particle damage. Conventional techniques include the use of low-density tin-doped foam targets, tin-doped water droplet targets, or shockwave punch-out foils, the addition of low impedance (Z) elements into solid density tin, the use of electric and magnetic fields, and the addition of a background gas. Nevertheless, all of these techniques suffer from serious drawbacks, including limited effectiveness (e.g., below industry requirements on ion dose to the optics), reduced conversion efficiency, and the addition of undesirable impurities and complexity.
For at least these reasons, it would be advantageous if an improved light source involving the generation of LPP(s) could be developed. It would in particular be advantageous if, in at least some embodiments, the system operated in a manner such that the amount of high kinetic energy debris, and consequent optic or other damage resulting from such debris, were reduced so as to increase the operational lifetime of the light source.